Knowing the amount of particulate matter or soot present in the exhaust stream of a diesel engine is important for determining the operational state of an exhaust gas treatment device, for example, a diesel particulate filter. A particulate matter sensor is often placed in the exhaust conduit in order to sense the amount of particulate matter present in the exhaust stream. The amount of particulate matter sensed by the particulate matter sensor may then be used, for example, to determine if the diesel particulate filter has failed and is therefore emitting a greater amount of particulate matter than is allowed.
A typical particulate matter sensor uses a simple resistive device as the sensing element. The sensing element typically includes a non-conductive substrate, most often alumina or zirconia, and two conductive electrodes that may be made of a precious metal. The two electrodes may be formed in a pattern with inter-digitized fingers that maximizes the perimeter between the two electrodes. When the sensing element is disposed in the exhaust stream of a diesel engine, particulate matter will deposit thereupon and cause a high resistance short between the electrodes, thereby lowering the resistance between the two electrodes. The more particulate matter that is allowed to collect on the sensing element, the more the resistance between the two electrodes will decrease. The sensing element may be provided with a heater on the side opposite the electrodes in order to clean soot off of the electrodes when desired.
It is known to orient the sensing element of a particulate matter sensor such that the electrodes face toward the upstream end of the exhaust conduit. This allows particulate matter present in the exhaust gases to impinge on the sensing face and adhere thereto. One problem with this arrangement is that large particles that impinge the electrodes may create undesirable spikes in the output signal of the particulate matter sensor. Particles that impinge the sensing face may also adhere only temporarily to the electrodes due to high exhaust flow velocity, thereby producing an undesirable output signal from the particulate matter sensor.
It is also known to dispose a shield over the portion of the sensing element that extends into the exhaust conduit. The shield is provided with a plurality of passages that allow the exhaust gases to impinge the electrodes which face toward the upstream end of the exhaust conduit. One problem with shields of this configuration is that the portion of the shield between the plurality of passages masks or prevents the exhaust gasses from impinging on all areas of the electrodes, thereby rendering these masked areas unused and ineffective for sensing particulate matter. This shield configuration also does not address the unsatisfactory output signal that may result from large particulate matter impinging the sensing face. This shield configuration also does not address the problem of particles adhering only temporarily to the electrodes due to high exhaust flow velocity.
Another problem common to prior art particulate matter sensors is that the sensing element is not inserted far enough into the exhaust stream in order to reach the center of the exhaust conduit where the exhaust gas mixture is most likely to contain particulate matter regardless of the radial location of a failure in the diesel particulate filter. A minimum exhaust conduit length of 7 to 10 times its diameter may be needed to mix a stream of particulate matter so that the particulate matter is distributed across the entire diameter of the exhaust conduit. If the exhaust conduit is 90 mm in diameter and the particulate matter sensor is placed near the exhaust conduit wall at a position diametrically opposite the location of the failure in the diesel particulate filter, it could take almost 1 meter to mix a stream of particulate matter coming out of the diesel particulate filter in order for the particulate matter sensor to sense the particulate matter. Placement of the particulate matter sensor 1 meter from the diesel particulate filter may be undesirable, for example, because the length of the exhaust conduit may be less than 1 meter.
It has been found that simply inserting the sensing element further into the exhaust conduit in order to place the electrodes near the center of the exhaust conduit is not effective for sensing the particulate matter present in the exhaust gases because of shortfalls not present in a particulate matter sensor having a sensing element placed near the wall of the exhaust conduit. The length of the sensing element is limited by its strength. Therefore, if the sensing element is inserted far enough into the exhaust stream for its electrodes to be located near the center of the exhaust conduit, the sensing element will be disposed completely within the exhaust conduit. This will cause the sensing element to be the same temperature as the exhaust gases and therefore will have less of a tendency for particulate matter to adhere to the sensing element. A sensing element placed near the wall of the exhaust conduit will tend to have a temperature lower than the exhaust because a portion of the sensing element will extend to the cooler environment outside of the exhaust conduit and will pull heat out of the portion of sensing element that is located in the exhaust conduit. Due to the thermophoretic effect, the particulate matter that is at a higher temperature than the sensing element will have a tendency to adhere better to the cooler sensing element. However, a particulate matter sensor with a sensing element placed near the wall of the exhaust conduit has the deficiencies mentioned earlier.
What is needed is a particulate matter sensor that can effectively bring particulate matter from the center of the exhaust conduit, where the particulate matter is most likely to be present, to the sensing element located near the wall of the exhaust conduit where it can be sensed. What is also needed is a particulate matter sensor that is less effected by large particulate matter.